The present invention relates to a method of manufacturing compound single crystals, including silicon carbide single crystal, that are useful as electronic materials. More particularly, the present invention relates to a method of manufacturing a compound single crystal (for example, a semiconductor) including silicon carbide single crystal having a low defect density or little crystal lattice deformation, which is advantageous in the manufacture of semiconductor devices.
Conventionally, the growth of silicon carbide has been divided into bulk growth by sublimation and thin film formation by epitaxial growth on a substrate. Bulk growth by sublimation permits the growth of hexagonal (6H, 4H, and the like), high-temperature phase polymorphic silicon carbide and the manufacturing of substrates of silicon carbide itself. However, numerous defects (particularly micropipes) are introduced into the crystal, making it difficult to increase the area of the substrate surface. By contrast, epitaxial growth on a single crystalline substrate affords better control of the addition of impurities, an enlarged substrate surface area, and reduction in the micropipes that are problematic when employing sublimation. However, in the epitaxial growth method, there is often a problem in the form of an increased density of stacking defects due to the different lattice constants of the substrate material and silicon carbide. In particular, since the silicon that is generally employed as the growth substrate has a high level of crystal nonconformity with silicon carbide, numerous twins and antiphase boundaries (APB) appear in the silicon carbide growth layer, becoming a source of leak currents and the like and compromising characteristics of silicon carbide as electronic elements.
K. Shibahara et al. have proposed a method of growth on a silicon (001) surface substrate (see FIG. 3) in which the surface normal axis is slightly tilted (in which an off angle is introduced) from the  less than 001 greater than  direction in the  less than 110 greater than  direction as a method of effectively reducing antiphase boundaries (Applied Physics Letter, Vol. 50, 1987, p. 1,888). In this method, slightly tilting the substrate causes a step at the atomic level to be introduced at equal intervals in a single direction. Thus, in vapor phase growth, epitaxial growth is imparted by step flow and the propagation of planar defects in the direction perpendicular to the steps (cutting across the steps) that are introduced is inhibited. Thus, as the thickness of the silicon carbide film increases, among the two antiphase regions in the film, the antiphase region expanding in a direction parallel to the step that has been introduced expands with priority over the antiphase region expanding in the direction perpendicular thereto, and antiphase boundaries are effectively reduced. However, as shown in FIG. 4, this method increases the step density of the silicon carbide/silicon substrate boundary, causing the unintended generation of antiphase boundary 1 and twins, and has a drawback in that it does not completely eliminate antiphase boundaries. In FIG. 4, 1 denotes the antiphase boundary occurring at a single atomic step in the silicon substrate, 2 denotes the junction point of antiphase boundaries, 3 denotes the antiphase boundary occurring at a silicon substrate surface terrace, xcex8 denotes the off angle, and xcfx86 denotes the angle (54.7xc2x0) formed between the Si (001) face and the antiphase boundary. Antiphase boundary 3 occurring at the silicon substrate surface terrace is eliminated at antiphase boundary junction point 2, but antiphase boundary 1 occurring at the single atom step in the silicon substrate has no junction and is not eliminated.
As a method of reducing such planar defects (twins, APBs) in silicon carbide, the present applicant has proposed the technique of eliminating planar defects propagating within silicon carbide by imparting undulations extending in a direction parallel to the silicon substrate surface and epitaxially growing silicon carbide on a substrate that has been processed with such undulations (Japanese Patent Application (TOKUGAN) No. 2000-365443 and Japanese Unexamined Patent Publication (KOKAI) No. 2000-178740). Imparting undulations to the silicon substrate has the effect of positioning off-tilted planes opposite each other on the silicon substrate as shown in FIG. 5. Thus, facing planar defects come together as shown in FIG. 6 and cancel each other out.
Based on this method, silicon carbide can be obtained in which planar defects are greatly reduced. However, when opposing planar defects come together and cancel out, one or the other of the planar defects remains and continues to propagate. In an actual undulated silicon substrate with an undulation interval of 2 micrometers and a silicon substrate plate thickness of 200 micrometers, for example, the remaining planar defect density is found to be a minimum of about 30/cm by simple calculation (assuming that one planar defect remains and propagates from each undulation). To completely eliminate these planar defects requires a plate thickness of 1.41 times the diameter of the silicon carbide (the thickness at which the spot at which the endmost planar defect ceases to propagate is not the growth surface). When employing vapor phase growth, the growth time required is excessive, rendering this thickness impractical.
Further, compound semiconductors other than silicon carbide, such as gallium nitride, are expected to serve as blue LEDs and power device materials. There have been numerous reports and examples of the growth of gallium nitride on silicon carbide substrates in recent years. This is because the use of silicon carbide as a base substrate in the growth of gallium nitride affords such advantages as facilitating the formation of electrodes, permitting ready heat dissipation, and facilitating handling and processing due to an identical crystal cleavage direction. However, there are problems in that it is difficult to achieve high-quality silicon carbide substrates with large surface areas, and although the difference in lattice constants is relatively small for silicon carbide, planar defects end up propagating in the gallium nitride growth layer due to boundary lattice nonconformity. Just as when growing silicon carbide on a silicon substrate, it is necessary to examine measures for eliminating defects.
The object of the present invention is to provide a method of manufacturing compound semiconductor single crystals such as silicon carbide and gallium nitride by epitaxial growth methods, that is capable of yielding compound single crystals of comparatively low planar defect density.
The present invention, which solves the above-stated problems, are as follows:
(1) A method of manufacturing compound single crystals in which two or more compound single crystalline layers identical to or differing from a single crystalline substrate are sequentially epitaxially grown on the surface of said substrate,
characterized in that at least a portion of said substrate surface has plural undulations extending in a single direction and second and subsequent epitaxial growth is conducted after the formation of plural undulations extending in a single direction in at least a portion of the surface of the compound single crystalline layer formed proximately.
(2) The method of manufacturing according to (1) wherein said compound single crystalline layer is a compound single crystal differing from the single crystalline substrate, and the compound single crystal constituting the compound single crystalline layer and the single crystal constituting the single crystalline substrate have similar space lattices.
(3) The method of manufacturing according to (1) or (2) wherein the direction of the plural undulations extending on the single crystalline substrate surface and the direction of extension of the plural undulations provided on the surface of the compound single crystalline layer formed on said substrate surface are either identical or perpendicular.
(4) The method of manufacturing according to any of (1) to (3) wherein the direction of extension of the plural undulations provided on the surface of the compound single crystalline layer provided adjacently to the growth direction is identical or perpendicular.
(5) The method of manufacturing according to any of (1) to (4) wherein said single crystalline substrate is single crystalline SiC.
(6) The method of manufacturing according to any of (1) to (5) wherein said compound single crystalline layer is single crystalline SiC, gallium nitride, aluminum nitride, or aluminum gallium nitride.
(7) The method of manufacturing according to any of (1) to (6) wherein said single crystalline SiC substrate is cubic SiC in which the basal plane is the (001) face, or hexagonal SiC in which the basal plane is the (11-20) face or the (1100) face.